The present invention pertains to the field of tuners with voltage-variable frequency determining elements.
Electronic tuners for television receivers conventionally comprise a tuned circuit including a varactor diode reverse biased to exhibit a capacitive characteristic and an inductor for each of the television bands. For example, in the United States, television receivers include tuned circuits including a varactor diode and an inductor for a low VHF band including channels 2 through 6, a high VHF band including channels 7 through 13 and a UHF band including channels 14 through 83. The tuned circuits are selectively enabled by bandswitching signals representative of the band in which a selected channel resides. The capacitive reactance of the varactor diode and thereby the frequency to which the tuned circuits are tuned is determined by the magnitude of a tuning voltage.
Many systems for deriving the tuning voltage are known. One of the simplest and therefore most economical types of these systems includes a potentiometer coupled between two points of potential. The tuning voltage is derived at a movable contact or wiper arm of the potentiometer and therefore is continuously variable. This permits a single tuning control for both coarse and fine tuning. In addition, such systems are rapidly tunable compared to so-called mechanical detent tuners.
Unfortunately, in general, the bands of tuning voltage corresponding to the three bands of the television tuning range are not consecutive but rather overlap each other. That is, the magnitudes of tuning voltage corresponding to the lowest frequency channels in the high VHF and UHF bands are lower than the magnitudes of tuning voltage corresponding to the highest frequency channels in the respective one of the preceding bands. Therefore, television tuning systems of this type in which a tuning voltage is generated at the movable contact of a potentiometer have typically not been able to continuously and consecutively tune a television receiver throughout the television tuning range.